Messages in this thread | | | Date | Thu, 28 Sep 2006 23:35:58 +0200 | From | Jan Kara <> | Subject | Re: [PATCH] Fix commit of ordered data buffers |
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Hello,
> On Tue, 2006-09-12 at 05:05, Jan Kara wrote: > > Hi Andrew, > > > > here is the patch that came out of the thread "set_page_buffer_dirty > > should skip unmapped buffers". It fixes several flaws in the code > > writing out ordered data buffers during commit. It definitely fixed the > > problem Badari was seeing with fsx-linux test. Could you include it > > into -mm? Since there are quite complex interactions with other JBD code > > and the locking is kind of ugly, I'd leave it in -mm for a while whether > > some bug does not emerge ;). Thanks. > > > > Honza > I also worked on it because I didn't know you were working on it until I > located the root cause and tried to check bugzilla. > > I reviewed your patch. > > + if (!inverted_lock(journal, bh)) { > + jbd_lock_bh_state(bh); > + spin_lock(&journal->j_list_lock); > + } > Should journal->j_list_lock be unlocked before jbd_lock_bh_state(bh)? It does not matter... The ordering of locking matters, ordering of unlocking does not.
> The fsx-linux test issue is a race between journal_commit_transaction > and journal_dirty_data. After journal_commit_transaction adds buffer_head pointers > to wbuf, it might unlock journal->j_list_lock. Although all buffer head in wbuf are locked, > does that prevent journal_dirty_data from unlinking the buffer head from the transaction > and fsx-linux from truncating it? Yes, it does. Because the buffers are locked *and dirty*. Nothing can clear the dirty bit while we are holding the lock and journal_dirty_data() also waits until it can safely write out the buffer - which is after we release the buffer lock.
> I'm not a journal expert. But I want to discuss it. > > My investigation is below (Scenario): > > fsx-linux starts journal_dirty_data and journal_dirty_data links a jh to > journal->j_running_transaction's t_sync_datalist, kjournald might not > write the buffer to disk quickly, but saves it to array wbuf. > Then, fsx-linux starts the second journal_dirty_data of a new transaction > might submit the same buffer head and move the jh to the new transaction's > t_sync_datalist. Yes, but this happens only after the buffer is removed from wbuf[] as I explain above.
> Then, fsx-linux truncates the last a couple of buffers of a page. > Then, block_write_full_page calls invalidatepage to invalidate the last a couple > of buffers of the page, so the journal_heads of the buffer_head are unlinked and > are marked as unmapped. > Then, fsx-linux extend the file and does a msync after changing the page content > by mmaping the page, so the page (inclduing the last buffer head) is marked dirty > again. > Then, kjournald's journal_commit_transaction goes through wbuf to submit_bh all > dirty buffers, but one buffer head is already marked as unmapped. A bug check is > triggerred. > > >From above scenario, as long as the late calls doesn't try to lock the buffer head, > the race condition still exists. > > I think the right way is to let journal_dirty_data to wait till wbuf is flushed. This actually happens in my fix too. And my fix has also a bonus of fixing a few other flaws... Otherwise your patch seems to be right.
Honza
-- Jan Kara <jack@suse.cz> SuSE CR Labs - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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